1. Field of Invention
The present invention relates to rolling apparatuses for bars. In particular, the present invention relates to a high-speed rolling apparatus for bars, which can provide simple, easy, and highly accurate tension-control. The tension is applied to the bars between a finishing mill and a sizing mill provided downstream from the finishing mill. The bars can have various shapes and can include reinforcing wire rods.
2. Description of Related Art
In general, bars are produced by rolling processes including rough rolling, intermediate rolling, finish rolling by a finishing mill including a plurality of roll stands, and a sizing rolling by a sizing mill including a plurality of roll stands.
In these bar rolling processes, the tension that is applied to the bars between each mill must be controlled so that breakage and buckling do not occur during rolling. Accurate tension-control is particularly important in high-speed rolling.
Methods for controlling tension are known, such as a method for controlling motor current of mill stands and another method that uses a looper.
A method for controlling motor current of mill stands is disclosed, for example, in Japanese Unexamined Patent Application Publication Nos. Sho-57-72716 and Sho-61-226108. In this method, the motor current applied for roll stand control is regulated so that the motor current is set to provide a tensionless state when the bar is engaged by a roll stand of the subsequent process. The motor current to provide a tensionless state is obtained by storing a current value before the roll stand of the following process engages the bar. This method is called the "current memory method".
However, it is very difficult to practice high-speed rolling by using the current memory method. For example, when the distance between each mill is 10 meters and the bar runs at a high speed of 100 m/s, the current for providing a tensionless state must be applied in less than 0.1 second, which is practically impossible to achieve.
Another known method is the "torque arm memory method." In this method, the tension applied to the bars is obtained from a torque arm value of the roll axle, which is obtained from a rolling torque obtained by a current value varying according to rolling resistance, and a rolling reactive force (actual value) detected by a detector of the mill for detecting rolling load. In the torque arm memory method, a looper sets the torque arm value to a proper value for controlling the bar tension directly.
However, in methods that use a looper, the looper cannot follow the high-speed transfer of a material such as a bar for control purposes, which is at more than 100 meters per second.
Moreover, it is difficult to use a rolling load detector, which is used in the torque arm memory method, in a planetary cross rolling mill (which includes conical rolls that rotate and revolve) generally used for rolling bars. It is difficult to use such rolling load detectors due to higher costs because precise measuring and controlling technologies are required to use them.